It requests more detailed information about the company's data protection policies, including questions about whether Google allows its own employees to access the content of users' emails.

Google responded to the report, stating that it gives third-party apps the option to request user data in order to function better. The tech giant also says it properly vets these companies with more access and that all users who had their emails read agreed to hand over their data in user agreements.

The committee questioned if the millions of Gmail users realized what they were giving up.

"While we recognize that third-party email apps need access to Gmail data to provide various services, and that users consent to much of this access, the full scope of the use of email content and the ease with which developer employees may be able to read personal emails are likely not well understood by most consumers," the letter states.

The senators gave Alphabet a July 24 deadline to provide answers to their questions.